This invention relates to the field of high temperature ceramic reactions and more particularly to an improved process for preparing barium titanate and related semiconductive materials by a continuous calcining process.
Barium titanate and perovskite -- related titanates and zirconates, when properly doped, are semiconductive materials which undergo solid phase changes at particular temperatures. Associated with these phase changes are very abrupt and large changes in the resistivity of the materials. The temperature at which such abrupt change occurs is referred to as the curie point and, because the change is positive with respect to temperture, the aforesaid titanates and zirconates are referred to as "positive temperature coefficient" materials. As a result of the resistance in temperature anomaly which they exhibit, these materials have been found useful as thermosensing elements in numerous diverse applications.
Among the various divalent cations which may be partially or totally substituted for barium in positive temperature coefficient titanate and zirconate compounds are calcium, strontium, lead, and tin. The partial or total substitution of the latter cations provides a semiconductive material whose curie point is in some cases higher and in other cases lower than pure barium titanate. The usefulness of these materials arises from the inclusion of minor proportions of various dopants such as lanthanide series rare earth metals, antimony and bismuth. The oxides of these metals are also useful as dopants. Other dopants or promoters such as iron, cobalt, manganese, ruthenium, vanadium, chromium, aluminum or silicon or te oxides thereof are also useful in very minor amounts for the purpose of increasing the extent of change in resistance at the curie point and sharpening the transition by increasing the slope of the resistivity vs. temperature curve in the neighborhood of the curie point.
The preparation of barium titanate and related positive temperature coefficient materials involves high temperature ceramic reactions which require careful proportioning of reactants and intimate intermixture thereof to assure complete reaction and stoichiometric balance in the product, factors which are important to a sharp and definitive response at the curie point. Because the reactants from which barium titanate is produced are solid at the reaction temperature, both the solid state diffusion-controlled reaction and the problem of separating unreacted material from the product render the realization of optimum properties difficult to achieve. In conventional solid state ceramic reaction processes, the raw materials tend to be inadequately mixed and additional contaminants are frequently introduced during mechanical mixing. Also, undesirable crystalline transformations such as anatase to rutile transformation of titanium dioxide may occur during the process, and the product of the process often fails to possess the desired physical and electrical properties.
Attempts have been made in the art to develop wet chemical processes for the production of barium titanate and related materials. One process which has previously been employed for the production of barium titanate is that described by Walsh in U.S. Pat. No. 2,988,422. In accordance with that process, a finely dispersed solution of titanium lactate and a barium compound is passed through a flame for volatilization of the solvent and thermal decomposition of the barium and titanium compounds to crystalline barium titanate. The product of this process is thus formed as a fine gas-borne dust and process equipment must consequently be adapted for handling substantial volumetric flow rates relative to the rate of production. The fine dust product also presents obvious collection problems and, in the case of toxic products such as lead titanate, the operation of this type of process may involve health hazards. Because of its exceptionally small particle size, moreover, the product of a direct flame reaction process may also be susceptible to discontinuous grain growth, with resultant formation of large individual grains in the fine grain material.
Another process known to the art for the production of barium titanate and related semiconductive materials is that described by Faxon et al. in U.S. Pat. No. 3,637,531. In accordance with the method of Faxon et al., one reactant solution is prepared containing the titanium chelate of triethanolamine, a second solution is prepared containing an alkaline earth salt such as barium acetate, the two solutions are combined to provide a common solution, and the common solution is heated at a low temperature to form a gel in which intimate intermixture of the alkaline earth salt and titanium compound are achieved. The gel is then heated at high temperature in an oxidizing or neutral atmosphere for the purpose of calcining the constituents of the gel and forming the desired titanate product. The method of Faxon et al. is a substantial improvement in the art, but this process involves heating with the process materials in a static state. If carried out continuously, the Faxon et al. process suggests that the feed materials be carried through the reaction zone in boats or other ceramic vessels which hold discrete quantities of static material. There has, thus, remained a need in the art for further improved methods of preparing barium titanate and those related titanates and zirconates which possess the advantageous electrical properties described above.